Image output device and image control method for smoothing image or increasing image resolution by use of same

ABSTRACT

The present invention comprises the steps of one-dimensionally converting, into a vector, a dot distribution around an attentional pixel in a K×L coordinate (K and L=odd numbers, M=L or K≠L) adjacent to the attentional pixel, specifically, in a 3×3 coordinate, 3×5 (5×3) coordinate or 3×7 (7×3) coordinate on the basis of rasterized data of the coordinate; and then smoothing the image or heightening an image density by correcting a location error of the attentional pixel on the basis of the vector information and the kind of attentional pixel (a dot or a blank). Thus, according to the present invention, the image patterns can be smoothed and the image resolution can be increased in a pseudo state with ease and high precision without complicating a circuit constitution by a simple judgement operation.

This is a continuation of application Ser. No. 08/067,212, filed May 26,1993.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

The present invention relates to an image output device for forming animage pattern in a dot matrix state, for example, a page printer such asa laser printer or a LED printer, or a CRT display, and an image controlmethod for smoothing an image or increasing an image resolution by theuse of the output device.

(ii) Description of the Prior Art

Heretofore, a laser printer has been known in which image patterns areformed in a dot matrix state on a photo-sensitive drum by repeatedlyphoto-scanning, in a main scanning direction and along buses on thedrum, laser beams which have been modulated on the basis of video dataserially output from an image controller.

Furthermore, a LED printer has also been known in which an arrayed LEDhead having LED elements linearly arranged in a main scanning directionis disposed along the bus of a photosensitive drum so as to confront thephoto-sensitive drum, and the LED elements are controlled to drive(burn) one line at one time or every one block of the line on the basisof video data and the photosensitive drum is relatively moved in asecondary scanning direction, thereby forming an image pattern in a dotmatrix state on the photosensitive drum.

In these kinds of printers, a processing for smoothing the image patternor a processing for increasing an image resolution is necessary in orderto obtain a sharper image, because the printers take a system in whichan optional letter or graphic form is formed by suitably depicting a dotimage pattern in an n×m dot matrix region on the basis of video data.

In the first place, the conventional technique of the processing forsmoothing the image pattern will be described.

For example, in the case of forming a slanted line or a curved line suchas "V" or "O", adjacent dots in the line are inconveniently formed inthe state of a step. Additionally, the crossing portion of a letter suchas "X" is made bold, because a plurality of dots are closely arranged,and in consequence, a printing quality deteriorates unavoidably.

In order to overcome such a drawback, a shift, i.e., a deviation of anattentional dot from the slant of an original line pattern is regardedas 1/2 of a distance between the attentional dot and a dot adjacentthereto, and presence or absence of the deviation of the attentionalpixel is confirmed by ORing/ANDing the attentional dot or a blank (thedot which may be the blank will be generically called "pixel") with anadjacent pixel.

The above-mentioned dot means the pixel which will be printed by theadhesion of a toner on one corresponding point in an n×m dot matrixregion, and the above-mentioned blank means the pixel showing abackground point on which any toner will not adhere in the matrixregion.

As described above, the deviation of the attentional dot which forms thestep is at most 1/2 of a distance between the two dots (in the case thatthe slant of a slanted line which is the image pattern is 45°). However,as the gradient of the slanted line is near to a vertical (horizontal)direction, a gap between the above-mentioned deviation and an originalimage increases. In other words, when the gradient of the slanted lineis near to the vertical or the horizon, it is very difficult to smooththe step of the slanted line even by ORing/ANDing the adjacent pixels.

The step of the slanted line always contains the adjacent dot and blankin the main scanning direction and/or the secondary scanning direction.Thus, as described in Japanese Patent Application Laid-open No.251761/1985, a technique has been suggested in which the blank adjacentto the attentional dot in the main scanning direction is replaced with adot having a small energy density (which will be the dot having a smalldiameter on a photosensitive drum) to smooth the step portion. In thistechnique, however, the dot having the small diameter is merely added tothe blank region adjacent to the attentional dot. Therefore, on a fineslanted or curved line, the dot-added portion becomes bold, which doesnot always lead to the improvement of the image quality. In addition,since this technique intends to add the dot to the blank in the mainscanning direction, a smoothing effect is low, as the gradient of theslanted or curved line decreases.

In order to overcome the above-mentioned drawback, U.S. Pat. No.4,847,641 (hereinafter referred to as "U.S. Pat. No. 641") has suggesteda technique in which the dot having a small diameter is added to theblank adjacent to the attentional dot and the attentional dot itself isalso narrowed (flattened) and apparently moved to one side, and theaddition of the small-diameter dot and the flattening of the attentionaldot are achieved not only in the main scanning (horizontal) directionbut also in the secondary scanning (vertical) direction, in other words,in the two directions of right/left and top/bottom.

Next, the constitution of this conventional technique will be brieflydescribed in reference to FIG. 9. In the first place, there are preparedfour compensating subcells 51a, 51b, 51c and 51d which are narrowed inthe main scanning direction and four compensating subcells 52a, 52b, 52cand 52d which are narrowed in the secondary scanning direction. Next,bit data (binary data arranged in a matrix state in a secondary scanningdirection, and generally, 1 and 0 correspond to the dot and the blank,respectively) corresponding to video data are successively seriallystored in an FIFO buffer 53 capable of storing the data every plurallines, and a plurality of upper, lower, right and left bit data adjacentto the attentional pixel are extracted through a sample window 54 andthen forwarded to a matching network 55. In this matching network 55,the bit map data present in the sample window 54 are compared to manytemplates 56 stored in the matching network 55. If the bit data accordwith the template 56, the compensating subcell 51a, 52a . . . forflattening the attentional dot and the compensating subcell 51a, 52a . .. for adding the flattened small dot to the upper, lower, right or leftblank adjacent to the attentional dot are selected from a subcellgenerator 57. On the contrary, if the bit data do not accord with anytemplates 56, the standard pixel is selected as it is. Afterward, thethus selected video data are serially output to a print engine driver 58to carry out printing. However, this conventional technique also hassome drawbacks. That is, in the practice of the technique, the twoselecting operations must be carried out to select the subcells of theattentional pixel and the adjacent other pixels. Additionally, the foursubcells are required for each of the horizontal direction and thevertical direction, and so the number of the templates necessary tocompare the bit map data present in the sample window 54 with thetemplates is (2⁴)×(2⁴)=256 in both the directions. In order to smooththe image patterns, application templates are further needful, and thetotal number of the templates is too large, which requireslarge-capacity memories for the large number of the templates and whichunavoidably complicates and consequently enlarges a circuit constitutionfor the comparing operation.

Next, some problems associated with the increase in an image resolutionwill be described.

As described above, among page printers such as laser printers and LEDprinters, the printers having an image constitution of a low dot pitchdensity, for example, 300 dpi are often employed because of low costs,compaction and the like. In recent years, however, for the purpose ofimproving image quality and resolution, printers in which the dot pitchdensity is heightened to, for example, 600 dpi have been suggested.

However, in order to obtain the pixel image of 600 dpi, bit map data inan image processing circuit must be also developed with the similarlyhigh density in data memory, which leads to not only the increase inmemory capacity, the complication of a hardware constitution and theincrease of the cost but also the retard of a processing rate.

In general, fonts of English and Japanese, already existent wordprocessing softs and other application softs have been made on the basisof 300 dpi, and therefore, in the case that the 600 dpi pixel image isemployed, they cannot be used as they are, with the result that theemployment of the 600 dpi pixel image is devoid of prevalence.

Accordingly, it has been attempted that the bit map data developed inthe state of 300 dpi are heightened to a high resolution level of 600dpi by using a data conversion circuit, and then fed to the print enginedriver.

For example, according to Japanese Patent Application Laid-open No.59362/1990, there are used 3 line memories having a memory capacitycorresponding to 600 dpi, a frequency multiplication circuit fordoubling a video clock frequency of 300 dpi and a discriminant circuitfor doing comparison/discrimination by logical sum, and the writing ofbit map data in one line memory is carried out in parallel with thereading of the data from other two line memories by the use of a clockconverted into 600 dpi in the frequency multiplication circuit. The thusread two data are ORed/ANDed in the discriminant circuit to output imagedata, and the image data of 600 dpi for each scanning line aresuccessively output to a print engine driver of a laser printer bycyclically moving the above-mentioned line memory every onecorresponding scanning line.

However, the foundation of the above-mentioned conventional techniqueresides in that the bit map data developed at 300 dpi in main andsecondary scanning directions are enlarged simply twice every onescanning line, and the high-density bit map data of 600 dpi are thenobtained by ORing/ANDing the attentional pixel with an adjacentreference bit. In other words, this technique is substantially equal tothat the clock is halved and the original image data are enlarged twiceby ORing/ANDing. This conventional technique can easily achieve theresolution enhancement of a slanted line at about 45°, but it cannotsmoothly achieve that of the slanted line close to a horizontal line ora vertical line.

In order to overcome the above-mentioned drawback, a technique has alsobeen suggested in which the original dot information of upper and lower5 lines×7 bits adjacent to the attentional bit is stored, and the dotinformation is ORed/ANDed by a logical circuit to heighten resolution inconsideration of a location error (Japanese Patent Application Laid-openNo. 60764/1990).

However, it makes the circuit constitution of the hardware extremelycomplex to OR/AND the two-dimensional 5×7 dot information which is asmuch as 35 bits.

Furthermore, in the conventional technique, the enhancement of theresolution is limited to a two-fold level, and it is impossible toachieve a higher level of the resolution.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a page printer oranother image output device which can smooth image patterns and canincrease an image resolution in a pseudo state with ease and highprecision without complicating a circuit constitution by a simplejudgement operation.

Another object of the present invention is to provide a method forsmoothing an image and improving the quality of dots themselves by whichthe high-quality image which can withstand the enlargement of smootheddata can be obtained.

Still another object of the present invention is to provide a method forforming a high-density image which can increase the density of imagedata in a pseudo state and can heighten resolution up to 600 dpi or ahigher level such as 900 dpi, 1200 dpi or 2100 dpi, a print enginedriver for 300 dpi being kept usable.

A further object of the present invention is to provide a method forforming a high-density image which can effectively heighten resolutionwithout being restricted by the gradient of an original slanted line orcurved line, in other words, even in the case of a slanted line having agradient close to a horizontal line or a vertical line.

The present invention is directed to a method for controlling an imagewhich comprises the steps of one-dimensionally converting, into avector, a dot distribution around an attentional pixel in a K×Lcoordinate (K and L=odd numbers, M=L or K≠L) adjacent to the attentionalpixel, specifically, in a 3×3 coordinate, 3×5 (5×3) coordinate or 3×7(7×3) coordinate on the basis of rasterized data of the coordinate; andthen smoothing the image or heightening an image density by correcting alocation error of the attentional pixel on the basis of the vectorinformation and the kind of attentional pixel (a dot or a blank).

In the present invention, for the rasterized data of the 3×3 coordinate,one piece of the vector information is enough. However, this piece ofthe vector information can only correct a location error of 1/2 of a dotpitch, and in particular, it cannot achieve the fine smoothing.

In the case of the 3×5 (5×3) coordinate or the 3×7 (7×3) coordinate, theprecise correction of the location error can be accomplished byobtaining the vector information on the basis of the rasterized data ofthe 3×5 (5×3) coordinate or the 3×7 (7×3) coordinate.

For example, as the rasterized data of the 3×7 (7×3) coordinate, thereare obtained three pieces of the vector information, that is, the firstvector information based on the rasterized data of the 3×3 coordinate,the second bit group adjacent to the outer periphery of the rasterizeddata of the 3×3 coordinate, i.e., the second vector information based onthe rasterized data of the 3×5 (5×3) coordinate, and the third bit groupadjacent to the outer periphery of the 3×5 (5×3) coordinate, i.e., thethird vector information based on the rasterized data of the 3×7 (7×3)coordinate. Above all, the first vector information based on therasterized data of the central component (the 3×3 coordinate) adjacentto the attentional pixel has the largest influence on the locationerror.

Thus, the three pieces of the vector information are multiplied byconstants and then summed up to obtain the total vector information, andin this case, the first vector information of the 3×3 coordinate ismultiplied by the largest constant. The thus obtained total vectorinformation is employed as the correction vector information for the 3×7(7×3) coordinate, whereby the fine correction control of the attentionalpixel location error, particularly the fine smoothing of the imagepattern can be achieved.

In the embodiment given hereinafter, the correction vector informationis obtained for the directional components of the K×L coordinate, i.e.,both of the total vector information of the 3×3, 3×5 and 3×7 coordinatesas ±X components and the total vector information of the 3×3, 5×3 and7×3 coordinates as ±Y components. However, an actual operation issimple. It is first judged whether a slanted line containing theattentional pixel is near to a vertical line or a horizontal line by the3×3 coordinate, and if it is near to the vertical line, the total vectorinformation in the ±Y axis direction of the 3×5 and 3×7 coordinates isobtained. If it is near to the horizontal line, the total vectorinformation in the ±X axis direction of the 5×3 and 7×3 coordinates isobtained. This procedure is enough to obtain the necessary information.

(Method for Smoothing an Image Pattern)

Now, a method for smoothing an image pattern will be described.

An image control method using the vector information, for example, amethod for smoothing an image pattern comprises preparing a plurality ofregisters in which smoothing data for the location error correction arestored; selecting the proper register on the basis of the vectorinformation and the kind of attentional pixel; forming a location errorcorrection pulse for forming the attentional pixel on the basis ofcorrection data in the selected register; and then outputting thecorrection pulse to a print engine driver, whereby the image can besmoothed.

Next, advantages of the smoothing method of the present invention willbe described in comparison with the above-mentioned U.S. Pat. No. 641.

As described above, according to U.S. Pat. No. 641, the rasterized dataof the coordinate present in a window 54 of 33 bits (3×7)+(7×3)-(9 ofoverlaps)! in all are randomly compared with many templates 56 stored ina network 55, and a corresponding template is selected from a subcellgenerator 57. In other words, the rasterized data of the 33-bitcoordinate are two-dimensionally randomly compared with the templates,and therefore, as described above, a tremendously large number of thetemplates are required to smooth the image pattern (the 2³³ templates atmaximum). Thus, large-capacity memories for many templates are required,and a circuit constitution for the comparison is also complicated and asa result, it must be enlarged, as described above.

On the contrary, the technique of the present invention is similar tothat of U.S. Pat. No. 641 in the employment of the window as therasterized data of the (3×7)+(7×3) coordinate, but it does not randomlyand two-dimensionally compare the data with the templates. In thepresent invention, the vector information of the dot distribution of theattentional pixel and the adjacent pixels in the window, in other words,the one-dimensional correction vector information of any of -X, +X, -Yand +Y directions in which a location error is present is firstobtained, and a register corresponding to, e.g., a compensating subcellis then selected on the basis of the vector information to smooth theimage pattern. Therefore, the one-dimensional processing, moredetailedly, the linear function processing is enough, and as a result,neither many templates nor any circuit constitution for the comparisonis necessary. In consequence, a problem of the conventional techniquesuch as the enlargement of the circuit constitution can be solved.

In addition, the register can be automatically selected on the basis ofthe vector information, for example, on the basis of 4K integers (-X,+X, -Y and +Y directions)×K (k≧L)! at maximum and the kind ofattentional pixel (a dot or a blank). In other words, the extremely finecontrol can be achieved only by preparing the 2K registers every onedirectional component (-X, +X, -Y or +Y direction).

Therefore, according to the present invention, the image pattern can besmoothed on the basis of the one-dimensional correction vectorinformation in any of the -X, +X, -Y and +Y directions having thelocation error by a simple judgement with ease and precision and withoutcomplicating the circuit constitution.

Moreover, the present invention intends to smooth the image pattern andsimultaneously to heighten the quality of the dots themselves, and thusthe high-quality image can be obtained which can sufficiently withstandthe enlargement of the data after the smoothing processing.

(Method for Increasing an Image Density)

Next, reference will be made to a method for forming a high-densityimage from low-density video data on the basis of the above-mentionedvector information.

The method for increasing an image density is characterized bycomprising the steps of selecting one suitable conversion register froma high-density conversion register group in which previously preparedM×M bit data for location error correction are stored, on the basis ofthe above-mentioned correction vector information and the bit kind ofattentional pixel (a dot or a blank); forming high-density video dataevery one scanning line on the basis of bit data read out from theselected register; and then outputting the video data to a print enginedriver in accordance with an M-fold video clock.

Next, an image output device which can smoothly heighten the imagedensity will be described, but for the simplification of explanation,reference will be made to a circuit constitution for increasing theimage density twice (M=2) from a low-density image of 300 dpi to ahigh-density image of 600 dpi in accordance with FIG. 7.

(1) In the first place, a means 42 is disposed which converts, into avector, a dot distribution around an attentional pixel in a 3×3coordinate including the attentional pixel on the basis of rasterizeddata of the coordinate.

(2) Next, a means 44 is disposed which selects one suitable conversionregister from a density doubling register group 45 in which previouslyprepared 2×2 data for location error correction are stored, on the basisof vector information obtained by the above-mentioned means 42 and thekind of attentional pixel (a dot or a blank).

In this case, in order to precisely correct the location error,{4×(M×M)-2} conversion registers 45a . . . , that is, in the case thatthe density is doubled, the 14 conversion registers 45a . . . arenecessary (counting in a case where all of 4 bits are "O" and a casewhere all of 4 bits are "1", the number of the conversion registers 45a. . . is 16).

(3) A pair of data memories 47A, 47B are disposed which store video datacorresponding to a nth scanning line and a n+1)th scanning line fromdata latched circuits 46, 46B to which 2 (M) bit data corresponding tothe respective lines have been directly or indirectly fed from theregisters 45a . . .

Each of the above-mentioned data memories 47A, 47B is required to have amemory region capable of writing the data of at least one scanning line.

(4) A shift register 48 is disposed which reads the above-mentioned pairof data memories 47A, 47B and then serially outputs high-density imagedata of corresponding scanning lines to a print engine driver, while thehigh-density image data are synchronized to a video clock having adensity which is twice as much as the density of an original videoclock.

The meaning of "2 (M) bit data have been directly or indirectly fed" inthe above paragraph is as follows. The read bit width of the datamemories 47A, 47B do not always accord with the data length of the M bitdata (2 bits in the above case). Therefore, the data latch circuit 46Bhaving a memory length corresponding to the bit width of the datamemories 47A, 47B is disposed, and the M bit data of the register 45corresponding to each scanning line are synchronized to the originalimage video clock and successively latched every M bits in the datalatched circuit 46B. The data corresponding to the bit width are thenOutputted from the data latch circuit 46B to the data memories 47A, 47B.

In the present invention, the rasterized data are not limited to theabove-mentioned 3×3 coordinate, and some coordinates are applicable.First, it is judged whether the 40° vector component of the attentionalpixel, i.e., the gradient of the slanted line pattern is near to avertical line or a horizontal line in the 3×3 coordinate. Next, if theslanted line is near to the vertical line, the total vector informationin ±Y axis direction in 3×5 and 3×7 coordinates is obtained, and if itis near to the horizontal line, the total vector information in ±X axisdirection in 5×3 and 7×3 coordinates is obtained.

In this case, the above-mentioned three pieces of the vector informationare multiplied by factors and then summed up to obtain the total vectorinformation, and at this time, the information of the 3×3 coordinate ismultiplied by the largest factor and that of the 7×3 or 7×3 coordinateis multiplied by the smallest factor. The thus obtained total vectorinformation is employed to obtain the fine location error informationfor the attentional pixel. In particular, the fine location errorinformation permits increasing the image density from 300 dpi to 900 dpior 1200 dpi, and in other words, the image density can be heightenedthree times or four times by the use of the fine location errorinformation.

Next, advantages of the image density enhancement according to thepresent invention will be described in comparison with Japanese PatentApplication Laid-open No. 59362/1990.

As described above, in the technique disclosed in Japanese PatentApplication Laid-open No. 59362/1990, the rasterized data of the35-pixel coordinate are randomly two-dimensionally logically compared tothe templates, and therefore the disclosed technique has the drawbackthat a large-scaled circuit constitution is required.

On the other hand, the present invention does not two-dimensionallycompare to the templates. According to the present invention, correctionvector information in any one-dimensional direction of the -X, +X, -Yand +Y directions having a location error is first obtained as in theabove-mentioned smoothing processing, and a previously prepared registeris selected on the basis of the vector information to increase an imagedensity, i.e., an image resolution. Therefore, a linearly functionalprocessing is enough, and the load of a circuit constitution and CPU canbe relieved. In addition, speed-up can also be achieved.

The selection of the register can also be automatically made on thebasis of an integer obtained from the correction information in anydirection of the -X, +X, -Y and +Y directions and a kind of attentionalpixel (a dot or a blank), and therefore extremely fine control can beachieved in consideration of locational correction only by preparing the{4×(M×M)-2} registers every one direction component.

Consequently, according to the present invention, the increase in theimage density in a main scanning direction and a secondary scanningdirection can be accomplished by a one-dimensional processing on thebasis of the correction vector information. Therefore, a high-qualityimage can be easily and precisely obtained without complicating thecircuit constitution by simple arithmetic processing, and since alocational correction function is employed, the obtained image cansufficiently withstand the enlargement of the data after the increase inthe image density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional view showing the fundamental principle of thepresent invention, and (A) is a fundamental constitutional view, (B) iscoordinate data necessary for the operation of (C), and (C) shows anoperational procedure for obtaining a correction vector information ofan attentional pixel.

FIG. 2 is a fundamental constitutional view of a printer incorporatedwith an image smoothing circuit which can be used in an embodiment ofthe present invention.

FIG. 3 is a block diagram showing the whole constitutional view of thesmoothing circuit of the present invention.

FIG. 4 are pulse waveforms illustrating kinds of image smoothing videodata produced in the smoothing circuit, and (A) shows the pulsewaveforms of a slanted line near to a horizontal line, and (B) and (C)show the pulse waveforms of slanted lines near to a vertical line.

FIG. 5 shows an image dot distribution formed on a photosensitive drumon the basis of the video data in FIG. 4, and (A) and (B) show the dotdistribution before the smoothing processing and after the smoothingprocessing, respectively.

FIG. 6 is a fundamental constitutional view of a printer incorporatedwith an image density doubling circuit which can be used in theembodiment of the present invention.

FIG. 7 is a block diagram showing the whole constitutional view of theimage density doubling circuit which can be used in the presentinvention.

FIG. 8 shows kinds of image density increasing registers which can beused in the image density doubling circuit.

FIG. 9 is a block diagram showing the whole constitutional view of aconventional image smoothing circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, the present invention will be exemplarily described in more detailin accordance with its embodiment in reference to drawings. However, itshould be understood that size, material, shape and relative arrangementof constitutional parts described in the embodiment do not intend tolimit the scope of the present invention and they are merely exemplary.

In the first place, reference will be made to procedure for obtaininglocation error information which can be used to smooth an image patternand to increase an image resolution in the present invention, togetherwith the description of the successive route toward the presentinvention.

As described above, when an original image containing a slanted line ora curved line is developed as an image pattern on a photosensitive drumin the form of a matrix coordinate, the original image is represented bya combination of vertical and horizontal dots containing a step in Xaxis and Y axis directions.

In the image pattern containing the step developed in the form of amatrix coordinate, a dot (0,0) at the step shown in FIG. 1 (A) isconsidered to be a dot containing an error, i.e., a deviation to beshifted in a right direction as compared with a position on which thedot should be actually present so that, for example, a slanted line maybe located on a coordinate determined by resolution.

The deviation from the central dot, i.e., the deviation of thecoordinate point (0,0)→a coordinate point (1,0) is obtained as asynthetic vector A₁ of the following two kinds of vectors A₂ and A₃ :

vector A₂ : point (0,0)→point (1,-1)

vector A₃ : point (0,0)→point (0,1)

Hence, A₁ =A₂ +A₃.

Next, how to obtain fundamental vector information regarding a 3×3 dotmap will be described.

That is, as shown in FIG. 1 (A), a 3×3 matrix is defined by acoordinate, and the pixel state (dot/blank) of the coordinate (X,Y) isdenoted by D(X,Y)=1 or 0. For example, when the attentional pixel is adot, it can be represented by D(0,0)=1.

When a correction vector corresponding to 1/2 of a distance between thedots is formed, the location error of the attentional dot D(0,0) islargest.

That is, the deviation from the attentional pixel is a synthetic vectorA₁ (=A₂ +A₃) obtained by summing up vectors.

However, in the case of the bit map information of the 3×3 coordinate, afirst step locational correction of the 1/2 of the dot pitch can only beachieved, and as the gradient of the original slanted line deviates from45° (as it approaches to horizon or vertical), the location error of theattentional dot increases.

(However, the double enhancement of the image density from 300 dpi to600 dpi can be sufficiently achieved by performing the first steplocational correction of 1/2 of the dot pitch in the 3×3 coordinate.)

For example, when the slanted line is near to the horizontal line or thevertical line, the attentional dot has the large location error owing tothe smoothing processing by OR/AND, and thus the large correction isrequired. This reason is that a ratio of the deviation of the dot in theX direction to that of the dot in the Y direction is low, or a ratio ofthe deviation of the dot in the Y direction to that of the dot in the xdirection is low.

Therefore, in the case that the rasterized data of the 3×3 coordinateare selected as the correction vector of the attentional dot of the dotsconstituting the slanted line near to the horizontal line, it isdifficult to obtain a sufficient effect.

Therefore, in the present invention, the rasterized data are enlarged inhorizontal and vertical directions to prepare a matrix of a 7×3coordinate and a 3×7 coordinate, and on the thus enlarged matrix, thecorrection vector is then obtained to carry out the control.

Theoretically, it is also possible that the correction vector isobtained by the use of the data of a matrix containing an N×3 coordinateand a 3×N coordinate in which N is 7 or more, to smooth the imagepattern. However, the employment of the matrix containing the 7×3coordinate and the 3×7 coordinate is preferable in view of theperformance of the print engine which is now used, as describedhereinafter.

Thus, how to obtain the correction vector of the matrix containing the7×3 coordinate and the 3×7 coordinate as shown in FIG. 1 (B) will bedescribed as follows.

First, the correction vector of the rasterized data of the 3×7coordinate regarding the attentional dot can be obtained by calculatingvectors of bold arrows in the 3×3 matrix, the 3×5 matrix and the 3×7matrix in FIG. 1 (C), and then summing up the three calculated vectors.

In FIG. 1 (C), each slanted dotted line is the gradient of an actualimage line.

Among the three vectors, the vector of the 3×3 matrix has the largestlocation error, and that of the 3×7 matrix has the smallest locationerror.

In order to facilitate the digital processing and in order to preciselycorrect the location error of the attentional pixel, the vector of the3×3 matrix, that of the 3×5 matrix and that of the 3×7 matrix aremultiplied by factors of, for example, 4, 2 and 1, respectively. On thebasis of the thus multiplied correction vectors and the kind ofattentional pixel (the dot or the blank), the location error of theattentional pixel is corrected to smooth the image pattern or toincrease the image density.

Next, how to obtain the correction vector of the 3×7 coordinate shown inFIG. 1 (C) will be described in more detail.

(1) In the first place, a first correction vector will be obtained fromthe 3×3 matrix as follows.

The pixels for giving correction vectors in -X, +X, -Y and +Y directionsare:

two pairs of a pair of D(-1,-1) and D(0,1) as well as a pair of D(-1,1)and D(0,-1) in the -X direction,

two pairs of a pair of D(1,1) and D(0,-1) as well as a pair of D(1,-1)and D(0,1) in the +X direction,

two pairs of a pair of D(-1,-1) and D(1,0) as well as a pair of D(1,-1)and D(-1,0) in the -Y direction, and

two pairs of a pair of D(1,1) and D(-1,0) as well as a pair of D(-1,1)and D(1,0) in the +Y direction.

Next, the correction vector of the 3×3 matrix will be obtained by addingthe conditions of the attentional pixel to the effective conditions ofthe correction vector.

Here, the vector having a size of 1 and either alone of the X componentand the Y component can be defined as a unit vector (X,Y) as follows

(X,Y)={(-1,0),(1,0),(0,-1),(0,1)}.

If DR₃ (X,Y) is regarded as the correction vector which functions in thedirection of the unit vector (X,Y) from D(0,0),

DR₃ (X,Y)={D(0,0)+D(X,Y)}/D(-X,-Y)}{D(Y,X)D(X-Y,Y-X)+D(-Y,-X)D(X+Y,Y+X)}.

Here, D(0,0)+D(X,Y) is an item referring to continuous conditions of aboundary. In this item, D(0,0) is connected with the attentional pixel,and D(X,Y) is a correcting directional component in the case that theattentional pixel is blank.

The item D(-X,-Y) in the above formula is an inhibition vector having acomponent opposite to a vector component in the (X,Y) direction.

In the above-mentioned formula, "/" and "+" mean logical NOT and logicalOR, respectively, and the operator of logical AND is omitted.

(2) Next, a second correction vector will be obtained from the 3×5 or5×3 matrix.

The pixels for giving correction vectors in -X, +X, -Y and +Y directionsare:

two pairs of a pair of D(-1,-2) and D(0,2) as well as a pair of D(-1,2)and D(0,-2) in the -X direction,

two pairs of a pair of D(1,2) and D(0,-2) as well as a pair of D(1,-2)and D(0,2) in the +X direction,

two pairs of a pair of D(-2,-1) and D(2,0) as well as a pair of D(2,-1)and D(-2,0) in the -Y direction, and

two pairs of a pair of D(2,1) and D(-2,0) as well as a pair of D(-2,1)and D(2,0) in the +Y direction.

Next, the correction vector of the 3×5 or 5×3 matrix can be obtained byadding the conditions of the attentional pixel to the effectiveconditions of the correction vector.

If the correction vector is a correction vector which functions in thedirection of the unit vector (X,Y) from D(0,0), the correction vectorCR₅ (X,Y) can be represented as follows:

CR₅ (X,Y)={CR₃ (X,Y)+C₃ (X,y)+S₃ (X,Y)}/D(-X,-Y){D(2Y,2X)D(X-2Y,Y-2X)+D(X+2Y,Y+2X)}

wherein C₃ (X,Y)=D(0,0)D(Y,X)D(-Y,-X) and S₃(X,Y)=D(X-Y,Y-X)D(Y,X)D(X+Y,Y+X)D(0,0)

wherein C₃ (X,Y) is a central component, and S₃ (X,Y) is a correctingdirectional component.

The item of CR₃ (X,Y) means that the correction vector of the 3×5 and/or5×3 matrix is effective when the correction vector of the 3×3 matrix ispresent.

CR₃ (X,Y)+S₃ (X,Y) is an item referring to continuous conditions of aboundary. In this item, CR₃ (X,Y) is the central component, and S₃ (X,Y)is a correcting directional component in the case that the attentionalpixel is blank.

The item D(-X,-Y) in the above formula is an inhibition vector having acomponent opposite to a vector component in the (X,Y) direction.

(3) Next, a third correction vector will be obtained from the 3×7 or 7×3matrix.

The pixels for giving correction vectors in -X, +X, -Y and +Y directionsare:

two pairs of a pair of D(-1,-3) and D(0,3) as well as a pair of D(-1,3)and D(0,-3) in the -X direction,

two pairs of a pair of D(1,3) and D(0,-3) as well as a pair of D(1,-3)and D(0,3) in the +X direction,

two pairs of a pair of D(-3,-1) and D(3,0) as well as a pair of D(3,-1)and D(-3,0) in the -Y direction, and

two pairs of a pair of D(3,1) and D(-3,0) as well as a pair of D(-3,1)and D(3,0) in the +Y direction.

Next, the correction vector of the 3×7 or 7×3 matrix can be obtained byadding the conditions of the attentional pixel to the effectiveconditions of the correction vector. If the correction vector is acorrection vector which functions in the direction of the unit vector(X,Y) from D(0,0), the correction vector CR₇ (X,Y) can be represented asfollows:

CR₇ (X,Y)={CR₅ (X,Y)+C₅ (X,Y)+S₅ (X,Y)}/D(-X,-Y){D(3Y,3X)D(X-3Y,Y-3X)+D(-3Y,-3X)D(X+3Y,Y+3X)}

wherein C₅ (X,Y)=C₃ (X,Y)D(2Y,2X)D(-2Y,-2X) and S₅ (X,Y)=S₅(X,Y)D(X-2Y,Y-2X)D(X+2Y,Y+2X)

wherein C₅ (X,Y) is a central component, and S₅ (X,Y) is a correctingdirectional component.

The item of CR₅ (X,Y) means that the correction vector of the 3×7 or 7×3matrix is effective when the correction vector of the 3×5 or 5×3coordinate matrix is present.

CR₅ (X,Y)+S₅ (X,Y) is an item referring to continuous conditions of aboundary. In this item, CR₅ (X,Y) is the central component, and S₅ (X,Y)is a correcting directional component in the case that the attentionalpixel is blank.

The item D(-X,-Y) in the above formula is an inhibition vector having acomponent opposite to a vector component in the (X,Y) direction.

(4) Calculation of a correction vector which acts on the attentional dot

The correction vector CR(X,Y) of the attentional dot obtained from allof the 3×3, 3×5, 5×3, 3×7 and 7×3 matrixes can be represented asfollows:

    CR(X,Y)=K.sub.3 CR.sub.3 (X,Y)+K.sub.5 CR.sub.5 (X,Y)+K.sub.7 CR.sub.7 (X,Y)(1)

wherein K₃, K₅ and K₇ are factors by which the correction vectors of therespective coordinates are multiplied.

The reason why these factors are necessary is that the influence of thecorrection vector on the attentional pixel is different. The influenceof the correction vector on the 3×3 coordinate is largest, and thus thefactors for the 3×3 coordinate, the 3×5 or 5×3 coordinate and the 3×7 or7×3 coordinate are, for example, 4, 2 and 1, respectively.

Then, the above-mentioned formula (1) is substituted by coordinatepoints of, for example, (-1,0), (1,0), (0,-1) and (0,1), whereby therecan be obtained the correction vectors of CR(-1,0) in the -X direction,CR(1,0) in the +X direction, CR(0,-1) in the -Y direction and CR(0,-1)in the +Y direction.

In the case that the factors of K₃, K₅ and K₇ are 4, 2 and 1, each ofCR(-1,0), CR(1,0), CR(0,-1) and CR(0,1) is 7 at maximum, because thecorrection vector information of each of CR₃ (X,Y), CR₅ (X,Y) and CR₇(X,Y) is 1 (an absolute value) at maximum.

(5) Selection of Correction Vector Information

Among the correction vectors in the -X, +X, -Y and +Y directions, thelargest correction vector information is employed by an optional judgingmeans, and the correction vector information is then converted intocorrection video data for a smoothing processing or density doublingvideo data for density enhancement every the corresponding attentionalpixel by the undermentioned procedure.

Next, the smoothing processing by the use of the correction vector willbe described.

A. Smoothing Processing

According to the calculation formula of the correction vector, at most 7pieces of the vector information can be obtained for each of CR(-1,0),CR(1,0), CR(0,-1) and CR(0,1), when they are multified by factors of,for example, 4, 2 and 1, and in other words, 7 kinds of the correctionvector information can be obtained. Furthermore, in the case that theattentional pixel is 1 (dot) or 0 (blank), the correction information isinverted when seen from the black side, and as a result, 14 kinds of thevector information can be obtained.

Correction dots for the smoothing processing can be formed as follows.For example, in the case of an LED printer and in a divide exposuresystem in which one pixel is divided into a plurality of subpixels inthe secondary scanning direction, followed by exposure, as disclosed inU.S. Pat. No. 641 of the present applicant, the formation of thecorrection dots can be achieved by dividing the attentional pixel intothe subpixels under control in accordance with a signal from theregister selected on the basis of the correction vector information andthe kind of attentional pixel, and then exposing the subpixels.Moreover, in the case of a laser printer, the formation of thecorrection dots can be achieved by modulating the pulse width of thevideo data in accordance with the register selected on the basis of thecorrection vector information and the kind of attentional pixel.

FIG. 2 is a fundamental constitutional view illustrating a hardwareconstitution for use in the laser printer regarding an embodiment of thepresent invention which can achieve the smoothing processing. In aprinter controller 1, as already known, the video data rasterized into300 dpi (300×300 dots per inch²) on the basis of the control of CPU 11are read out from a font RAM 12 in correspondence with image signalsfrom a host computer 2, and they are successively written in a videomemory 13. In the video memory 13, the video data corresponding to onepage or one band width are developed in the form of bits, and thenconverted into serial data synchronized to a video clock, and the serialvideo data are output to a print engine driver 20. For example, the beamof a semiconductor laser 21 is scanned via a polygon mirror 22 at apitch interval corresponding to 300 dpi under ON/OFF modulation controlon the basis of the video data to form an image pattern corresponding tothe video data on a photosensitive drum 23, and printing is then carriedout by a suitable electrophotography.

In the present embodiment, a smoothing processing circuit 3 shown inFIG. 3 is interposed between the printer controller 1 and the printengine driver 20, and the smoothing processing can be achieved bymodulating the video data.

That is, the smoothing processing circuit 3 contains a latch circuit 32for latching 7×7 dot data in order to form 3×7 and 7×3 windows byreading/writing between the latch circuit 32 and a 7-line memory 11comprising SRAM 31, an arithmetic circuit 33 for calculating thecorrection vector information of the attentional pixel on the basis ofthe above-mentioned calculation formula, and a conversion circuit 34 forconverting, into a correction dot, the attentional pixel output from theprint controller 1 in accordance with the correction vector informationoutput from the arithmetic circuit 33. Reference numeral 35 is aregister receiving section in which various conversion registers 15A-15Lfor converting the attentional pixel into the correction dot on thebasis of the correction vector information are received.

Next, the operation of the above-mentioned circuit will be brieflydescribed. The 7-line memory 31 is constituted of SRAM capable ofaccommodating the video data of seven scanning lines and one scanninglines, i.e., 8 scanning lines. The video data which are serially outputfrom the print controller 1 in synchronization with a video clock aresuccessively stored in a first bank 11a of the 7-line memory 11 in turn.After the video data for one scanning line are stored in the first bank11a, the same procedure as described above is repeated to store thevideo data for a second scanning line in a second bank 11b and the videodata for a third scanning line in a third bank 11c in turn.

When the video data for 7 lines are stored in the first to the seventhbanks, the video data stored in corresponding addresses of the first tothe seventh banks are read out in turn on the basis of a latch signal bythe utilization of a time until the data of the address corresponding tothe eighth scanning line are output from the print controller 1, andthese data are successively loaded into the 7-line shift registerconstituting the data latch circuit 32.

As a result, in the 7-line shift register constituting the latch circuit32, there are stored and arrange the rasterized data (7×7) of acoordinate comprising the attentional pixel, 3 lines in front of and 3lines in the rear of the attentional pixel and 3 bits on right and leftsides of the attentional pixel, while the content of the data issuccessively changed owing to data transfer from the 7-line memory 11,and the rasterized data in the coordinate can be utilized to carry out avector arithmetic processing in the arithmetic circuit 33.

In the arithmetic circuit 33, as shown in FIG. 1 (B), window data of 3×7(X,Y) and 7×3 (X,Y) are taken out from the rasterized data (7×7) of thecoordinate constituting the 7-line shift register, and ±X directionvector information, ±Y direction vector information or ±45° vectorinformation are obtained in accordance with the correction vectorcalculation formula mentioned in the previous paragraph (1). Next, theattentional pixel is converted into correction video data correspondingto the above-mentioned correction vector information on the basis of theobtained vector information corresponding to the attentional pixel andthe kind of attentional pixel (the dot or the blank) in theundermentioned conversion circuit 34, and the video data are then outputto a print engine driver 3.

In this case, the location error of the attentional pixel to becorrected is at most 1/2 of a distance between the attentional pixel andan adjacent dot, and therefore even if the information of the correctionvector is identical, the correction information is inverted as seen fromthe dot side in the case that the attentional pixel is the dot or theblank (e.g., the location error of 70% of the blank is achieved bygiving a dot corrected to 30%).

Therefore, for example, in the case that 7 kinds of the correctionvector information are obtained from the window data of the 3×7 (X,Y)and 7×3 (X,Y), and in the case that the attentional dot is the dot orthe blank, the correction information is inverted as seen from the dotside, and therefore 14 kinds of the information can be obtained.

However, as described above, even when the fine control of the 14 kindsis carried out, the constitution is only complicated in fact.Furthermore, even if the laser beam is subjected to the ON/OFF controlunder fine pulse control, a precise gradation cannot be obtained owingto the scatter of an output luminous intensity in the printing section.

Thus, according to the constitution of the present invention, the 14kinds of the correction information are classified into 6 kinds, wherebythe attentional pixel to be corrected is replaced with the 6 steps ofthe correction information.

In this case, in the present embodiment, 6 kinds of the conversionregisters 15A to 15L are prepared every one of direction components in aCR(-1,0) (-X direction), CR(1,0) (+X direction) and CR(0,-1/ 0,1) (+Ydirections). That is, 18 kinds of the conversion registers are preparedin all.

Incidentally, the correction dot is generally formed by modulating thepulse width of the attentional dot. However, in the case of the laserprinter, the scan line of the laser beam is directed in a horizontaldirection, and therefore the dot shift in horizontal ±X directions iseasy, but the shift in the vertical ±Y directions is difficult. For thisreason, in the embodiment of the present invention, the processing iscarried out by pulse control so that the size of the dot may be changed,without changing the dot position in the vertical direction.

For example, as shown in FIG. 4 (I), it is first judged in theconversion circuit 34 whether a curved line (inclusive of a straightline) containing the attentional dot is a slanted line of 45° or morenear to a vertical line, or a slanted line near to a horizontal line. Ifthe curved line is near to the horizontal line, correction data selectedfrom the undermentioned registers A to F on the basis of the attentionalpixel {dot: (D(0,0=1)), blank: (D(0,0)=0)} information and thecorrection vector information {CR(x,y)=1-7} are parallelly written in ashift register 34a of a 16-bit memory in the conversion circuit 34. Thedata are serially output from the shift register 34A on the basis of aclock having a frequency 16 times as much as that of 300 dpi so as tooutput corrected video data to the print engine driver 3 as shown inFIG. 4, whereby the image dots (toner dots) to be formed on aphotosensitive drum can be corrected. Thus, when the correction videodata shown in FIG. 4 (I) are used to smooth an image pattern, a printline comprising the slanted line or the curved line near to thehorizontal line as shown in FIG. 5 (A) (A) can be replaced with a printline smoothed in a secondary scanning direction as shown in FIG. 5 (A)(B).

    ______________________________________                                        (Data content of conversion registers A to F)                                 ______________________________________                                        A:     1111111001111111                                                                              B:    1111110000111111                                 C:     1111100000011111                                                                              D:    1111000000000111                                 E:     1110000000000111                                                                              F:    1100000000000011                                 ______________________________________                                    

On the other hand, in the case that the original slanted line or curvedline is near to a vertical line, it is judged whether theabove-mentioned vector information CR(x,y) is the vector informationCR(1,0) in a positive (+X direction) or the vector information CR(-1,0)in a negative (-X direction), and correction data selected fromcorresponding registers of G to L and M to R on the basis of theattentional pixel {dot: (D(0,0=1)), blank: (D(0,0)=0)} information andthe correction vector information (CR(x,y)=1-7) are parallelly writtenin the 16-bit shift register 34a in the conversion circuit 34. The dataare serially output from the shift register 34a on the basis of a clockhaving a frequency 16 times as much as that of 300 dpi so as to outputcorrected video data to the print engine driver 3 as shown in FIGS. 4(II) and (III).

As a result, as shown in FIG. 5 (B), a print line comprising a slantedline or a curved line near to a vertical line such as (A) can beconverted into a print line smoothed in a main scanning direction asshown in (B) on the side of the print engine driver 3.

    ______________________________________                                        (Data content of conversion registers G to R)                                 ______________________________________                                        G:     1111111111111000                                                                              H:    1111111111100000                                 I:     1111111110000000                                                                              J:    1111111000000000                                 K:     1111100000000000                                                                              L:    1110000000000000                                 M:     0001111111111111                                                                              N:    0000011111111111                                 O:     0000000111111111                                                                              P:    0000000001111111                                 Q:     0000000000011111                                                                              R:    0000000000000111                                 ______________________________________                                    

B. Density Increasing Processing

A density increase processing is also carried out on the above-mentionedvector information, and for example, in the case that an image densityis doubled from 300 dpi to 600 dpi, the location error correction of thefirst step of 1/2 of a dot pitch is enough, and therefore the mapinformation of a 3×3 coordinate is sufficient.

B1) Calculation of Correction Vector

The correction vector of the 3×3 coordinate matrix can be represented asfollows.

(X,Y)={(-1,0), (1,0),(0,-1),(0,1)}

CR₃ (X,Y)={D(0,0)+D(X,Y)}/D(-X,-Y)}{D(Y,X)D(X-Y,Y-X)+D(-Y,-X)D(X+Y,Y+X).

In this case, the Y component or the X component contains no locationerror, and therefore in the case that the image density is doubled from300 dpi to 600 dpi, among the conversion registers shown in FIG. 8,

a register 45F is selected, when CR₃ (0,-1)=1,

a register 45g is selected, when CR₃ (1,0)=1,

a register 45h is selected, when CR₃ (0,1)=1, and

a register 45i is selected, when CR₃ (-1,0)=1.

Furthermore, Q(X,Y)=D(X,0)·D(0,Y)}·/D(-X,0)·/D(0-Y) is defined as avector in a 45° direction which acts on D(0,0). At this time, (X≠0 andY≠0).

The correction vector CR_(3A) (X,Y) for doubling the image density isdetermined by the use of Q(X,Y), and CR_(3A) (X,Y) can be represented asfollows:

    CR.sub.3A (X,Y)=D(X,-Y)·D(0,0)D(-X,Y)-/D(X,-Y)·/ D(X,-Y)·/D(0,0)/D(-X,Y)}Q(X,Y)                   (1)

In the above-mentioned formula, "/" and "+" mean logical NOT and logicalOR, respectively, and the operator of logical AND is omitted. Whensubjected by the coordinate of the above-mentioned CR_(3A)(X,Y)={-1,0,1}, relations between the pieces of the correction vectorinformation of the respective directional components and the conversionregisters are as follows.

A register 45b is selected, when CR_(3A) (-1,-1)=-1,

a register 45j is selected, when CR_(3A) (-1,-1)=1,

a register 45c is selected, when CR_(3A) (1,1)=-1,

a register 45k is selected, when CR_(3A) (1,1)=1,

a register 45d is selected, when CR_(3A) (1,-1)=-1,

a register 45l is selected, when CR_(3A) (1,-1)=1,

a register 45e is selected, when CR_(3A) (1,1)=-1, and

a register 45m is selected, when CR_(3A) (1,1)=1.

Incidentally, in the present embodiment, the correction vectorinformation is determined on the basis of the rasterized data of the 3×3coordinate, and therefore the vector information for judging whether theattentional direction component is near to the vertical direction or thehorizontal direction is not necessary. However, in the case that theprecise locational correction control is carried out by the use of therasterized data (window) of the 5×5 or more coordinate, e.g., therasterized data of the 7×7 or more coordinate, it is first judgedwhether the 45° vector component of the attentional pixel is neat to thevertical direction or the horizontal direction on the basis of the 3×3coordinate. If the slanted line is near to the vertical direction, thetotal vector information in the ±Y axial directions is determined on thebasis of the pieces of the vector information of the 3×5 and 3×7coordinates alone. Furthermore, if the slanted line is near to thehorizontal direction, the total vector information in the ±X axialdirections is determined on the basis of the pieces of the vectorinformation of the 5×3 and 7×3 coordinates alone. They are similar tothe case of the above-mentioned smoothing processing.

B2) Preparation of Density Increasing Register Group 45

The density increasing register group 45 is previously prepared inaccordance with resolution. However, in the case of the doubleconversion as described above, one pixel is enlarged twice in both ofthe main scanning direction and the secondary scanning direction, andtherefore 2×2 conversion registers are necessary.

As shown in FIG. 8, as the conversion registers, there are prepared aconversion register 45a of a: (00,00), conversion registers 45b to 45eof 45b: (10,00), 45c: (01,00), 45d: (00,10), 45e: (00,01) in the casethat the high-density pixel of "1" is rotated, conversion registers 45fto 45i of 45f: (11,00), 45g: (01,10), 45h: (00,11), 45i: (10,01) in thecase that the dot pixel of "11" is rotated, conversion registers 45j to45m of 45j: (11,10), 45k: (01,11), 45w: (10,11), 45m: (11,01) in thecase that the dot pixel of "111" is rotated, and conversion registers45a to 45n of 14 of the conversion register 15n of n: (11,11).

B3) Selection of Conversion Register

Among the correction vectors in the -X, +X, -Y and +Y directions, thelargest correction vector information is employed, and density doublingregisters 45a . . . are selected on the basis of the correction vectorinformation and the attentional pixel (a dot or a blank).

Then, the one corresponding conversion register is selected from theselected conversion register group 45a-45n, and density increasing videodata are formed every one scanning line on the basis of the bit dataread out from the register group 45a-45n. Next, the video data areoutput to a print engine driver on the basis of a 2-fold video clock,whereby an image having a doubled density of 600 dpi can be formed everyone scanning line. Its hardware constitution will be described inreference to FIGS. 6 and 7.

B4) Hardware Constitution

FIG. 6 is a fundamental constitution view illustrating the hardwareconstitution regarding the embodiment of the present invention fordoubling the image density. In a printer controller 1, as already known,the image data rasterized into 300 dpi (300×300 dots per inch²) are readout from a font RAM 12 in accordance with the image signals from a hostcomputer 2 in synchronization with a video clock, and then successivelywritten in a video memory. In the video memory 13, the video datacorresponding to one page or one band width are developed into an image,and they are then output from the video memory to a density doublingcircuit every one scanning line in synchronization with a video clock onthe basis of a horizontal synchronizing signal. In the density doublingcircuit, the high-density video data doubled in the main scanningdirection and the secondary scanning direction are output to a printengine driver 20 in accordance with a video clock having a velocitytwice as much as that of a video clock producer having a 2-fold density,whereby printing of the high-density video data is carried out on thebasis of the video data by photoscanning at a pitch intervalcorresponding to 600 dpi, while for example, laser beams areON/OFF-controlled.

Incidentally, on the side of the print engine driver 20, the lightningtime of the semiconductor laser 21 becomes 1/2 by the 2-fold densityvideo clock, but the scanning width in the main scanning direction canbe maintained similarly to the case of 300 dpi by doubling therotational frequency of a polygon mirror 22. Furthermore, the density ofthe scanning can be doubled in the secondary direction by halving therotational frequency of a photosensitive drum 23. As a result, ahigh-density image of 600 dpi can be printed. The detailed descriptionof this constitution will be omitted because of being known.

In the present embodiment, the double increase of the image density onthe basis of the above-mentioned system is realized by interposing adensity doubling circuit 4 shown in FIG. 7 between the printercontroller 1 and the print engine driver 20.

That is, the density doubling circuit 4 is composed of a latch circuit42 comprising a three-line shift register for latching the video data of3 scanning lines, a vector information converting circuit 43 forcalculating the above-mentioned vector information from the 3×3 videodata around the attentional pixel output from the latch circuit 42, aregister selecting circuit 44 for selecting one corresponding conversionregister from a density doubling register group 45 in which previouslyprepared 2×2 data for the location error correction are stored, on thebasis of the correction vector information output from the conversioncircuit 43 and a kind of attentional pixel (the dot or the blank), aregister storing portion 45 in which the above-mentioned 14 convertedregisters 15a-15m are stored, a data latch circuit 46 for n lines and adata latch circuit 46B for n+1 lines having 8-bit memory lengthcorresponding to a bit width of the undermentioned data memories 47A,47B, a pair of data memories 47A, 47B for storing pixel datacorresponding to the n scanning lines and the n+l scanning lines, whilethe 8-bit data are taken in from the above-mentioned latch circuits 46,46B in turn,.a shift register 48 which parallelly takes in the data fromthe data memories 47A, 47B, synchronizes the data to a high-densityvideo clock corresponding to 600 dpi and serially outputs thedensity-doubled video data to the print engine driver 20, and a controlcircuit 49 for controlling the output of the data memories 47A, 47B andthe shift register 48.

Each of the above-mentioned data memories 47A, 47B is required to have amemory region capable of writing the data of at least one scanning line.

Next, the operation of the above-mentioned circuit will be brieflydescribed. The video data stored in a corresponding address of SRAM 14in which the corresponding video data for 4 lines are stored aresuccessively read out from the video memory on the basis of a latchsignal which synchronizes to the video clock, and they are thensuccessively loaded into the data latch circuit 42 comprising thethree-line shift register.

As a result, in the three-line shift register constituting the latchcircuit 42, there are stored three scanning lines of the attentionalpixel line, one line in front of and one line in the rear of theattentional pixel by data transfer from the SRAM 14, while its contentis successively renewed. Thus, the coordinate data of (3×3) around theattentional pixel can be taken out from the shift register to permit theformation of the above-mentioned correction vector information in thevector information conversion circuit 43.

In the vector information conversion circuit 43, the 3×3 (X,Y)coordinate data from the above-mentioned latch circuit 42 are taken out,and ±X direction vector information, ±Y direction vector information andif necessary, ±45° vector information are obtained on the basis of thecorrection vector calculation formula shown in the previous paragraph1). Then, corresponding conversion registers 15a-15m are selected fromthe register storing portion in the undermentioned register conversioncircuit 44 on the basis of the correction vector informationcorresponding to the attentional pixel from the above-mentionedinformation and the kind of attentional pixel (the dot or the blank).

After the conversion registers 15a-15m are selected, among theabove-mentioned registers, the upper 2 bits and the lower 2 bits areparallelly loaded into the data latch circuit 46A for the n lines andthe data latch circuit 46B for the n+1 lines in synchronization with thevideo clock of the original image, respectively.

Every time the 8-bit data are latched in the data latch circuit 46B bythe repetition of the above-mentioned operation, these data areparallelly loaded into the respective data memories 47A, 47B, and thedensity-doubled video data corresponding to 600 dpi are stored in thedata memory 47A for the n lines and the data memory 47B for the n+1lines, respectively.

On the basis of an output control signal from the control circuit 49,the high-density video data in the data memory 47A for the n lines areparallelly loaded into the shift register 48, and the data memory forthe n scanning lines are serially output from the shift register 48 tothe print engine driver 20, while synchronized to the density-doubledvideo clock of 600 dpi. After the output, the high-density video data inthe data memory 47B for the n+1 lines are parallelly loaded into theshift register 48, and the above-mentioned operation is then repeated.

While the high-density image data for the n+1 scanning lines areserially output from the shift register 48, the density of the videodata for the next original scanning line is increased, and then theabove-mentioned operation is repeated.

What is claimed is:
 1. An image control method for use in an imageoutput device for forming a dot matrix image based upon rasterized bitmap data, the dot matrix image having a smoothness and an image density,the method comprising:identifying an attentional pixel having a locationerror and an identity as at least one of a dot pixel or a blank pixel,identifying a dot distribution in a K×L coordinate around theattentional pixel, where K and L represent odd numbers, obtainingrasterized data of the K×L coordinate, converting the dot distributionaround the attentional pixel into vector information on the basis of therasterized data, correcting the location error of the attentional pixelon the basis of the vector information and the identity of theattentional pixel as at least one of a dot pixel and a blank pixel,providing a plurality of registers capable of performing at least thestep of correcting the location error of the attentional pixel,selecting at least one of the plurality of registers on the basis of thevector information and the identity of the attentional pixel as at leastone of a dot pixel and a blank pixel, selecting, on the basis of thecorrection vector information and the identity of the attentional pixel,one register from the plurality of registers in which correction data ofM bits for smoothing are stored, where M represents an integer, writingcorrection data read out from the register in parallel in a shiftregister having at least an M-bit memory, and serially outputting videodata for smoothing in accordance with a clock having a frequency M timesgreater than a video clock to a print driver, wherein the K×L coordinatedefines an -X axis direction, a +X axis direction, a -Y axis directionand a +Y axis direction, and further comprising:obtaining vectorinformation on the basis of rasterized data of a K×L coordinate adjacentto the attentional pixel, where K≧L, and providing a register group forsmoothing in which a maximum of 2K registers are provided for each axisdirection.
 2. The method of claim 1 wherein the step of converting thedot distribution into vector information comprises:obtaining a firstpiece of vector information in the -X axis direction, the +X axisdirection, the -Y axis direction and the +Y axis direction, determiningwhich of the obtained first pieces of vector information is largest, andwherein the step of correcting the location error of the attentionalpixel comprises correcting the location error of the attentional pixelon the basis of the largest of the obtained first pieces of vectorinformation.
 3. The method of claim 1 further comprising the stepsof:determining whether a vector axis direction component of theattentional pixel in a 3×3 coordinate is relatively nearer to a verticaldirection or a horizontal direction, obtaining correction vectorinformation in at least one of the -Y axis direction and the +Y axisdirection when the vector axis direction component is relatively nearerto the vertical direction, obtaining correction vector information in atleast one of the -X axis direction and the +X axis direction when thevector axis direction component is relatively nearer to the horizontaldirection, and correcting the location error of the attentional pixel onthe basis of the vector information.
 4. The method of claim 1 forincreasing image density, comprising:selecting, on the basis of thecorrection vector information and the identify of the attentional pixel,a corresponding conversion register from a density increasing registergroup in which M×M bit data for location error correction are stored,wherein M represents an integer, forming high-density video data everyone scanning line on the basis of bit data read out from the selectedconversion register, and outputting the video data to a print enginedriver in accordance with at least one of an M-fold video clock.
 5. Animage output device for a page printer, comprising:means for convertinginto a vector a dot distribution around an attentional pixel in a K×Lcoordinate adjacent to the attentional pixel on the basis of rasterizeddata of the coordinate, where K and L represent odd numbers and K≧L,means for selecting correction vector information having a maximumlocation error from among pieces of vector information in a -X axisdirection, a +X axis direction, a -Y axis direction and a +Y axisdirection, means for selecting, on the basis of the vector informationand an identity of the attentional pixel as at least one of a dot pixeland a blank pixel, a corresponding register from a plurality ofregisters in which at least one of correction data for smoothing animage pattern and correction data for increasing an image density arestored, means for forming video data of at least one of a smoothed imagepattern and an increased image density on the basis of correction dataread out from the selected register, means for selecting, on the basisof the vector information and the identity of the attentional pixel asat least one of a dot pixel and a blank pixel, a corresponding densityincreasing register from a plurality of density increasing registers inwhich M×M bit data for location error correction are stored, where Mrepresents an integer, means for forming video data having an increasedimage density every one scanning line on the basis of the bit data readout from the selected density increasing register, M data memories forstoring video data corresponding to n to (n+M-1) scanning lines whilesuccessively taking in M-bit data corresponding to each scanning line,where n represents an integer, a plurality of M data latch circuitslocated between the selected register and the data memories, the M datalatch circuits having a memory length corresponding to a bit width ofthe data memories, whereby the M-bit data corresponding to each scanningline from the selected register is synchronized to an original videoclock and latched in the respective data latch circuits every M bits,and data corresponding to the bit width are output to the data memoriesvia each data latch circuit.
 6. The device of claim 5, comprising:meansfor selecting, on the basis of the vector information and the identityof the attentional pixel as at least one of a dot pixel and a blankpixel, a register from among a plurality of registers in which M-bitcorrection data for a smoothing process are stored, where M representsan integer, and a shift register having an M-bit memory for writing inparallel correction data read out from the register, whereby video datafor a smoothing process serially output from the shift register inaccordance with a dock having a frequency 16 times as much as M of avideo clock can be output to a print engine driver.